1. Field of the Invention
The invention relates to cutting elements for rotary drill bits, and of the kind comprising a facing table of superhard material having a front face and a rear surface bonded to the front surface of a substrate which is less hard than the superhard material and rim material coating the edges of the facing table.
2. Description of the Related Art
Such cutting elements usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a pocket in the body of the drill bit. The carrier is often also formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier. Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a pocket in the bit body, without being first brazed to a carrier. As is well known, the body of the drill bit itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process.
Cutting elements of the above-described kind are often in the form of circular or part-circular tablets. Each cutter is so mounted on the bit body that a portion of its periphery defines a cutting edge which acts on the surface of the formation being drilled. In the case of a circular cutter the cutting edge will be provided by a curved stretch of the circular periphery of the cutter. In some locations on the drill bit, such as in the gauge region of the bit, however, the cutting element will be formed with a straight cutting edge across part of its periphery to act on the formation.
Such cutting elements are subjected to extremes of temperature and heavy loads, including impact loads, when the drill is in use down a borehole. It is found that under drilling conditions spalling of the diamond table can occur, that is to say the separation and loss of diamond material over the cutting surface of the table. Such spalling usually spreads from the cutting edge, probably as a result of impact forces. The spalling reduces the cutting efficiency of the element, and in severe cases can lead to delamination, that is to say separation of the diamond table from the substrate.
In order to increase the lifetime or wear-resistance of superhard cutter elements it has been suggested to coat the superhard layer with layers of softer material. In U.S. Pat. No. 5,049,164 for example, a cutter element is described having a multi-layer metal coating. Further disclosure of coated cutter elements can be found in U.S. Pat. Nos. 5,135,061, and 5,833,021.
The known coating layers are usually applied to pre-fabricated cutting elements using plating techniques, vapor deposition techniques, sputtering, vacuum deposition, arc processes or high velocity spray processes. It is therefore seen as an object of the present invention to provide cutter elements where the facing table of superhard material is at least partially coated with a softer material. More specifically it is seen as an object of the invention to provide such cutter elements without additional coating step in the production process of the cutters.
According to the invention there is provided a preform cutting element, for a rotary drill bit, including a facing table of superhard material having a front face and a rear surface bonded to the front surface of a substrate which is less hard than the superhard material, the facing table having edges between the front face and the rear face, the edges being at least partially coated with a layer made of material less hard than the superhard material but different from the substrate material, and being high pressure bonded to the edges.
According to another aspect of the invention there is provided a drill bit comprising a body and a plurality of cutter elements as described above.
In a preferred embodiment a cutter element in accordance with the present invention has a uniform outer diameter prior to mounting the cutter element in the drill bit with edges of the facing table being at least partially coated with the coating material. In a variant of this embodiment the edges of facing table are essentially completely covered with the coating material.
In yet another preferred embodiment the coating material is bonded to both the superhard facing table and the substrate.
As defined herein, high-pressure bonding means a joining of two materials by applying high pressure and high temperature to the interface between the two materials. This process is known as such and widely used in the industry. In fact it is seen as very advantageous to provide the coating during the same high pressure and high temperature step that establishes the bond between the facing table and the substrate.
The materials for the coating layer are preferably chosen to be softer than the substrate material. Preferred materials for the coating layer are bondable to diamond and must be able to withstand the temperature and pressure to which it is subjected in the press during the diamond synthesis. Suitable coating material may be molybdenum (Mo), tantalum (Ta), niobium (Nb) and metallic alloys thereof.
Preferred material for the substrate are tungsten carbide or carbides of one of the group IVB, VB, or VIB metals of the periodic table, or alloys thereof, or other suitable materials and may contain a suitable binder material such as cobalt, nickel, or iron.
The superhard material is synthetic or natural diamond, cubic boron nitride or wurtzite boron nitride, and may contain binder material as described above.
The invention provides cutting elements that have a rim or annular jacket of soft compliant metal surrounding the superhard material. The compliance affords protection to the cutter element during insertion into the drill bit body and use thereof. Compared to previous alternatives, the rim material can be applied with only minor modification of the existing production methods for basic cutting elements. No extra step of plating or deposition is required.